1. Field of the Invention
The present invention relates to cables, and more particularly to cables having a plurality of conductors which are disposed in groups of three bundles each having three strands.
2. Description of the Prior Art
The push-pull transmission of digital signals by way of pairs of conductors is advantageous in that different ground potentials do not interfere with each other at the transmitting and receiving stations. However, the great demand for conductors is disadvantageous in the parallel transmission of a plurality of bits. As is known, an improvement of the ratio of items of information which can be simultaneously transmitted, to the total number of physical conductors from 0.5 bit to 0.75 bit results from the formation of so-called phantom circuits each of which consists of two double conductors. Furthermore, while maintaining the push-pull principle, it is possible to increase the number of the potential stages on the conductors of the conductor bundles having three and more conductors, to above two and thus to further improve the ratio of the items of information which can be transmitted to the number of conductors (cf German patent application No. P 29 39 252.7).
In a three-conductor bundle, the transmission capacity, for example, increases theoretically to the value 2.5. All three conductors have the same surge impedance relative to one another. On the other hand, in a four-conductor bundle, the surge impedances of adjacent and diagonal conductors usually differ from one another.
The further increase in the transmission capacity is achieved by combining three three-conductor bundles in one cable. Therefore, a fourth three-conductor arrangement which is to be referred to as a three-conductor phantom arrangement is obtained. In this manner, up to 10 bits (4.times.2.5 bit) can be transmitted via nine conductors.